[petsc-dev] snes/examples/tutorials/ex27.c

[Matthew Knepley]

On Tue, Nov 15, 2011 at 5:43 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Tue, Nov 15, 2011 at 17:37, Matthew Knepley <knepley at gmail.com> wrote:
>
>>   b) FormFunction and FormJacobian in a SNES could be used by an
>> enclosing TS to do pseudo-timestepping
>
>
> This can't work. What equation would the FormFunction and FormJacobian
> correspond to?
>

What are you on? It is F(x) = 0. This embedding will not work for all
problems, but it will work for many, and be easy. If you
have a more complicated system, by all means us TS, however I am pointing
out that there are useful regimes where this
kind of embedding would work.

   Matt


> The TS *defines* the equation that needs to be solved. The user provides
> information according to an interface. The TS interface is more general,
> and really is required for transient things. The SNESFormFunction isn't
> enough because you haven't identified positivity and constraints. Add that
> transient term to all variables equally and you are going to see some
> confusing stuff like huge conservation error and incorrect boundary
> conditions, such that you could easily end up on the wrong branch or
> mis-diagnose a limit cycle versus steady state.
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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